This invention concerns a method to eliminate the play between chocks and the relative support blocks in four-high rolling stands, and the relative device, as set forth in the respective main claims.
The invention is employed to minimize and even eliminate the value of the play and gaps between the chocks and the uprights of the housings, or between the chocks and the supporting bearing elements, or gibs, during the cross-over movement of the rolls (pair crossing).
The state of the art includes the rolling technique of pair crossing, wherein the working rolls, and possibly the relative back-up rolls, are crossed over so as to obtain a wide field of adjustment of the crown of the rolls, a better control of the profile of the rolled stock during the processing step and therefore a better quality final product.
During the crossover of the rolls, and because of it, a moment of flection is generated on the chocks of the back-up rolls; this moment of flection is determined by the misalignment of the vertical thrust forces which the rolled stock impresses on the rolls with respect to the load exerted by the hydraulic pressure means acting on the chocks of the said back-up rolls.
In other words, according to the angle of crossing of the rolls, the position of the horizontal resultant of the force of the hydraulic pressure means and the force transmitted by the rolled stock to the relative working roll, and from this to the back-up roll, may even be positioned outside the supporting planes between the chocks of the back-up rolls and the relative support blocks-gibs.
When this happens, the chock is no longer correctly supported and a flection torque is generated.
This flection torque generates an increase, on one side, in the forces which are exerted between the support element or gib and the relative chock.
This fact, with the same coefficient of friction between the sliding elements included between the chock and the supporting element, leads to an increase in the forces of friction.
Moreover, this turnover component generated by the flection torque tends to make the chock rotate on the vertical plane with respect to its longitudinal axis and, should there be an excessive gap, or a gap which has not been pre-set, between the chock and the housing, or between the chock and the support block, it can cause the edges of the chock itself to tip up against the relative bearing element.
This unplanned space or play between the chock and the bearing element can be caused by various factors, including the absence of compensation means or an imperfect functioning of such compensation means as are present.
This space or play in any case must always be guaranteed during the design step, even if only at a minimum value, due to the contraction of the space of the housing stressed by the rolling force.
This contraction of the space is due to the deformation under load of the structure of the housing which leads to the deformation of the uprights which bend, to values in the order of one or two millimetres, and curve inwards at the centre because of the extremely high rolling loads, causing zones of maximum proximity of the chock and the relative gib.
This play must therefore be planned in such a way that, when the rolling forces which deform the housing of the stand are at their highest values, there is no jamming of the chocks and the relative gibs. The greater the horizontal contraction of the uprights of the housing is, the greater the play must be.
An incorrect value of the play can lead to a risk of blows and impacts during the crossover displacement of the chock and relative gib or bearing block.
All this leads to incorrect functioning, risks of jams and damage, inaccurate control of the thickness and profile of the rolled stock, and therefore the final products are not of optimum quality and the sliding surfaces between the chocks and the gibs are subject to premature wear.
Moreover, the very presence of play between the chocks of the working rolls and the relative support blocks can lead to an inaccurate control of the angle of crossing of the rolls.
This leads to errors and imperfections in the technological control of the rolling process, with negative consequences on the adjustment of the profile and the thickness of the strip.
Furthermore, the presence of unplanned play can cause vibrations to start, caused by the horizontal movement of the working rolls in the direction of rolling.
The interaction of possible horizontal vibrations of the working rolls with the rolling process itself can generate self-excited linear vibrations, caused by the fact that the coefficient of rolling friction is a function of the relative speed of the rolls and the material being rolled.
Document JP-A-05-293518 describes a method to adjust the crossing angle between the working rolls in a four-high rolling stand according to the rolling load.
It provides to continuously monitor the value of the rolling load by means of a measuring device provided for this purpose, and to send the relative signal to a processor which adjusts in feedback the drive of the hydraulic jacks which act on the chocks of the working rolls.
This document does not teach to minimize the play between the chocks and the relative support blocks as the rolling load varies.
Documents JP-A-56-074310, JP-A-56-074311 and JP-A-56-074312 refer to devices to control on-line the thickness of the strip based on the control of the deformation of the stand under working conditions and with a crossing angle which is not zero.
In practice, these devices provide to calculate the actual deformation of the stand by continuously measuring the actual rolling force and the crossing angle, and taking into account the working parameters such as width and thickness of the strip.
According to these calculations, the pressure means on the stand are consequently adjusted so as to maintain the thickness of the strip at the values established by the working specifications.
The evolution described in JP""311 provides to control the actual position of the chocks, rather than the crossing angle, in such a way as to compensate any difference between the center of the crossing angle of the rolls and the vertical median plane of the stand.
These documents, like JP""518, do not provide to compensate the play between the chock and support blocks according to the rolling load.
Document JP-A-08-294713 shows a method to annul the play between the chocks of the working rolls and the back-up rolls and the relative support blocks in a four-high rolling stand, wherein hydraulic compensation cylinders are provided, on the inlet side of the stand, associated with position transducers.
The position transducers monitor the actual position of the chocks and supply this value to the relative hydraulic cylinders, which are thus able to take the chocks, at the outlet side of the stand, into contact with the relative support blocks.
This solution, although it is efficacious in itself, does not take into account the variation of the rolling load, and is simply directed to compensate wear on the sliding parts, which increases with time.
Document JP-A-59-087914 describes an embodiment which is substantially the same as the previous one.
The present applicants have designed, tested and embodied this invention to overcome this serious disadvantage which businessmen in the field have long complained of, and also to obtain further advantages.
The invention is set forth and characterized in the respective main claims, while the dependent claims describe other characteristics of the main embodiment.
The purpose of the invention is to provide a play compensation method, and a device connected thereto, suitable to obviate those problems which derive from the partial rotation of the chock of the back-up rolls with respect to the housing, or the support element, due to the turnover component caused by the eccentricity between the rolling thrust and the thrust of the hydraulic pressure system.
In other words, the invention proposes to optimize the value of the lateral play which is created between the chock and the relative support element during the crossing of the rolls, taking into account the entity of the horizontal contraction of the uprights of the housing during the rolling passes, in such a way that the turnover component which is inevitably generated can be at least partly compensated and so that in any case it does not create the aforesaid problems.
Another purpose of the invention is to minimize the vibrations of the working rolls on the horizontal plane during the rolling passes which start themselves off because of the play between the chocks and the support blocks.
A further purpose of the invention is to reduce and even eliminate the play between the chocks and the relative support blocks in order to minimize the technological problems due to inaccuracies in the control of the crossing angle which affect the control of the profile and the thickness of the strip being rolled.
According to the invention, the pressure signal from the load cell which monitors the thrust exerted by the hydraulic pressure means acting on the chock of the back-up roll is monitored and sent continuously to a processing and control unit.
Depending on the hydraulic pressure imparted to exert the rolling load, this signal provides information which makes it possible to define the entity of the theoretical contraction of the space of the housing due to the flection of the uprights, and therefore the entity of the consequent play, during the condition of contraction, between the chock and the relative support element.
Depending on the pressure signal, appropriately processed, the processing and control unit commands the activation of appropriate actuators which act on the support blocks so as to bring them near the relative chocks and at least optimize the lateral play between the chock and the support element to a desired value.
According to a variant, the value of contraction of the space defined by the rolling stand housings is calculated in advance, when the stand is inoperative, according to the expected value of the rolling force.
According to this pre-calculated value and the crossing program of the rolls during the rolling passes, the reference values of the position of the support blocks of the chocks are defined, in such a way that the play between the chocks and the blocks is substantially zero during rolling.
The rolling program can be pre-set or can be a consequence of the rolling conditions which occur on each occasion.
According to a further variant, the position of the support blocks with respect to the relative chocks is adjusted in such a way as to obtain an over-compensation of the play during rolling.
In other words, a support block is compressed, on one side, against the relative chock with a pre-set clamping force in order to be sure that no accidental movements are generated on the horizontal plane of the working and back-up rolls.
The value of the clamping force is defined so that the product of the clamping force and the coefficient of friction between the chocks and the relative support blocks, which supplies the value of the hysteresis of the hydraulic pressure means acting on the chock of the back-up roll, is less than a defined value.
In a preferential embodiment, the maximum threshold value is in the order of 20 tonnes, as the sum of all the vertical friction forces.
According to the invention, the displacement imparted to the support blocks takes place according to continuous values.
According to a variant, this displacement takes place when necessary, every time the contraction of the housing uprights makes the play between chock and support block exceed a maximum threshold, or is below a minimum limit threshold.
According to another variant, in cooperation with the chocks and the relative support blocks there are reciprocal distance monitoring means; these means serve to continuously control the distance between the chock and the relative support block as the rolling thrust varies.
These means supply control and reference data to the system.
The crossover displacement means which achieve this compensation, according to a variant, can be actuators attached to the stationary housing of the rolling stand and act on one side only.
According to a variant, the actuators are arranged symmetrically on opposite sides of the respective chocks.